The present invention relates to dielectric ceramics for use in dielectric layers of multilayer ceramic capacitors; and more particularly, to a dielectric ceramics with core-shell grain structures capable of providing favorable B temperature characteristics, wherein various shell portions having different functions can be adaptably arranged around a core in a manner suitable for obtaining desired characteristics.
When manufacturing multilayer ceramic capacitors having desired B temperature characteristics by using dielectric ceramics principally composed of barium titanate (BaTiO3) , it has been considered to be essential that crystal grains constituting the dielectric ceramics have a core-shell grain structure, wherein the core-shell grain structure includes a ferroelectric core and a paraelectric shell encompassing the core. The core-shell grain structure in the dielectric ceramics is obtained by using such additives as Mg and rare earth elements.
In manufacturing conventional dielectric ceramics having the core-shell grain structure, Mg and rare earth elements are added simultaneously to the dielectric ceramic material containing therein, e.g., BaTiO3 as a main component and diffused together into grains to form shells thereof. Moreover, no measure has been taken to control the distribution of Mg and the rare earth elements in the grains, resulting in Mg diffusion substantially deep into the dielectric grains.
Therefore, the conventional dielectric ceramics including dielectric grains having core-shell grain structures to improve B temperature characteristics may not be adaptably controlled to have required properties. The thickness of the dielectric layers has been continuously reduced to obtain an ever-increasing capacitance of multilayer ceramic capacitors, necessitating various quality requirements in such scaled down dielectric layers. However, the conventional core-shell grain structure cannot effectively meet such various quality requirements for the multilayer ceramic capacitors.
It is, therefore, an object of the present invention to provide multilayer ceramic capacitors with an improved performance and reliability by adaptively tailoring the shell structure of ceramic grains according to the required characteristics.
In accordance with one aspect of the present invention, there is provided a dielectric ceramic comprising:
dielectric ceramic grains having BaTiO3 as a major component thereof, a portion of the dielectric ceramic grains having a ferroelectric core and a paraelectric shell into which Mg and a rare earth element are diffused, the shell being located at least on a part of a surface of the core,
wherein the shell includes at least two shell portions having different components diffused thereinto, respectively.
In accordance with another aspect of the present invention, there is provided a multilayer ceramic electric part comprising the dielectric ceramic.
In accordance with still another aspect of the present invention, there is provided a method for manufacturing the multilayer ceramic electric part comprising the steps of:
producing a ceramic powder mixture having ceramic particles, the producing step including the step of mixing MgO and a substance containing a rare earth element with a BaTiO3 based dielectric ceramic material; and
removing portions of surfaces of the ceramic particles